The present invention pertains to new improved components for thermoplastic adhesives intended to cross-link slowly after application.
Thermoplastic adhesives are compositions intended to assure the bond between diverse materials. These adhesives are in the form of solid, stiff compositions at room temperature, but they melt when the temperature is raised and then develop adhesive properties in their melted state and continue to adhere strongly to the supports on which they were deposited upon returning to the solid state.
Generally, thermoplastic adhesives are compositions based on a thermoplastic polymer, e.g., EVA (ethylene-vinyl acetate copolymer), APAO (atactic poly .alpha.-olefin) and thermoplastic rubber, and which also contain, depending on the case:
(i) tackifier resin, a substance which is defined by its own specific physical and chemical properties, notably its compatibility in the hot state with polymer bases, and by the thermomechanical properties that it induces in the polymer mixtures in which it is contained, i.e., an improvement in their wettability and fluidity, an increase in their glass transition temperature T.sub.g and a reduction in their modulus;
(ii) plasticizers, substances that have the role of reducing the glass transition temperature T.sub.g and of making the mixture more pliable;
(iii) waxes and/or paraffins, which act as simple charges in the solidified material, which can accentuate the elastic performance and improve the mechanical performance, and as solvents in the melted product, which decrease the viscosity;
(iv) oils, which decrease the viscosity and improve the pliability at room temperature; and/or
(v) mineral charges, fillers (chalk), or pigments (titanium oxide).
Thermoplastic adhesives are extensively employed in industry, e.g., in packaging, furniture, labeling, binding and plumbing, where they are appreciated for their moderate price, ease of use, quick setting, lack of solvent and, generally, their low toxicity. Nevertheless, they have two major defects. On the one hand, their mechanical resistance is low and, on the other hand, their heat performance is very limited. These defects restrict their use to nonstructural bonding, i.e., to the bonding of materials that themselves have low mechanical resistance, such as paper, thin plastic films, cardboard and non-woven materials. Their mediocre heat performance is the corollary of their thermoplasticity, and originates in their composition itself, based on thermoplastic polymers such as ethylene/vinyl acetate copolymers, thermoplastic rubbers and atactic propylene, and limits their use to a temperature range of circa -30.degree. C. to +60.degree. C.
Bonding with a thermoplastic adhesive is carried out by depositing the hot (circa 160.degree. C.) product which has low viscosity (viscosity between circa 0.1 and 100 Pa.s) on the support, then almost immediate counterbonding with the second support. Setting is effected by simple cooling, by passage through a phase transition: crystallization and/or glass transition. This system is perfectly reversible and if the adhesive is reheated, it remelts. But even before remelting, it loses some of its strength which is manifested by a tendency to creep, which can result in the relative displacement of the bonded pieces or even their total detachment. This drawback can even be manifested with certain very pliable adhesives at room temperature.
For several years, chemists have explored many approaches in attempts to resolve this defect. These approaches can be grouped into bicomponent products and monocomponent products. The term "bicomponent" means a formulation which is necessarily presented in the form of two products that must be mixed together at the moment of use, because the mixture itself is reactive and its properties develop as soon as the mixture is created. As its name indicates, a "monocomponent" product is comprised of a single product which is stable when stored, at least for a reasonable period of time. The bicomponent product approach, notably based on epoxides, is difficult and implementation is delicate. The monocomponent approach is more fertile with monocomponents with high melting points or monocomponents that can be cross-linked by heat, by the oxygen in the air, by irradiation (UV, electron jet, etc.), by the moisture in the air or the supports to be bonded.
Improvement in heat performance by selecting thermoplastic components with high crystallization temperatures has given birth to thermoplastic adhesives based on polyamides and polyesters. These products, which remain expensive, have the drawback that they can only be applied at high temperatures and therefore undergo noteworthy thermal degradation and, in addition, the bonding quality is mediocre under ordinary application conditions, probably because the crystallization of these products takes places too quickly and is associated with poor wetting.
Chemical cross-linking can be created with heat, rather like the cross-linking of rubber by means of vulcanization. The chemical process of this process is extremely variable. A peroxide path is known with monomers and/or copolymers with potentially reactive residual double bonds, e.g., acrylic copolymers, or the urethane-acrylates from the Basenden Company. The product, which has a phase transition at circa 40.degree.-80.degree. C., is applied at circa 60.degree.-100.degree. C. and cross-linked at high temperature (circa 180.degree. C. for 10 minutes). Another approach that has been followed is that of polycondensation of EVA-type polymers modified by an hydroxyacrylate, which are cross-linked at circa 180.degree. C. for 10 minutes with a blocked isocyanate. Cf. the French patent application published as No. FR 2,616,155 (ATOCHEM) and European Patent Application No. 0 302 620 (EXXON CHEMICAL). These approaches are restrictive. They impose a high-temperature curing step, contrary to most applications. In addition, these products can begin the cross-linking process as they are applied, which creates the risk of damaging the application equipment.
The formulations based on chemical cross-linking using the oxygen from the air use alkyl boranes. They are more experimental than industrial in nature.
UV irradiation cross-linking was proposed by Dynamit Nobel with polyesters that are sensitive to UV radiation, which can be applied around 50.degree. C. SHELL employs the same principle with its KRATON 1320 X. This aproach is only applicable to the cross-linking of thin layers of product and its application is very specific (e.g., use for coating products).
Cross-linking using moisture has been exploited for many years by adhesives based on polyurethane-polyols with a low melting point, i.e., a T.sub.m between 30 and 50.degree. C. and/or with a high glass transition temperature, i.e., a T.sub.g- 40.degree. C. and +35.degree. C. (cf. notably European Patent No. 0 107 097, FULLER). This approach is also implemented with silicone mastics and thermoplastic adhesives based on EVA grated with a silanol (cf., notably British Patent No. 2 197 326, SWIFT). For these latter products, the hot stability is very mediocre and there is a very intense increase in the viscosity when the product remains exposed for more than three hours at circa 150.degree. C. This is obviously a major constraint in the manufacture and use of thermoplastic adhesives.
There have also been proposed adhesives based on EVA/liquid polyurethane prepolymers/tackifier resin mixtures in which the PPU prepolymer (the abbreviated term "PPU prepolymer" is used to designate the polyurethane prepolymers) is a conventional elastomer base that is almost liquid at room temperature and has excess free --NCO groups that make it cross-linkable with water. The PPU prepolymers are obtained in a known manner by reaction of an excess of polyisocyanate monomer with polyol monomers.
Studies have shown that in order to obtain proper cross-linking, the level of PPU prepolymer in the formulation must reach circa 30% by weight. The product is then highly softened and its cohesion at room temperature is considerably below that of a conventional thermoplastic adhesive as long as cross-linking has not been performed, which requires circa 24 hours.